This invention is primarily directed to anti-submarine operations in which the sonobuoys are dropped from aircraft to the surface of the ocean and hydrophones suspended from the buoys convert acoustical signals from the submarines to electrical signals. These signals are transmitted from the buoys to the aircraft, where they are processed to locate the submarine, which is then demolished or disabled by means of explosive charges dropped into the water.
Modern submarines, however, are characterized by both high speed and great maneuverability. The sonobuoys must therefore transmit signals which can be quickly processed to accurately locate the targets, so that the latter can be tracked closely enough to bring depth charges or other explosive devices within lethal range. High sensitivity is another requirement in present day systems. For example, where echo ranging is used, with the sonobuoys picking up signals generated by a suitable underwater source and then reflected from the submarine to be tracked, the acoustical signals are often highly attenuated by the time they are picked up, particularly if the system is to be operable over a large area. Also, if a passive system is to be used, with the sonobuoys detecting acoustical energy emanating from the submarine itself, high sensitivity is required because of the relative quietness with which modern submarines are capable of operating.
The cost, size and weight of the sonobuoys is an important factor, especially when they are to be carried by an aircraft, since they are expendable and a large number may be used in a given operation. In prior units the direction of the target has been ascertained by comparing the amplitudes of the electrical outputs of a number of hydrophones disposed in an array of sonobuoys. Accuracy of an arrangement of this type requires both individual calibration of all the hydrophones and extensive precautions against changes in their characteristics. These factors add considerably to the expense of the system, particularly where high accuracy is required.